|Publication number||US7525746 B1|
|Application number||US 11/836,050|
|Publication date||Apr 28, 2009|
|Filing date||Aug 8, 2007|
|Priority date||Mar 25, 2003|
|Also published as||US7116504, US7262928, US7286312|
|Publication number||11836050, 836050, US 7525746 B1, US 7525746B1, US-B1-7525746, US7525746 B1, US7525746B1|
|Original Assignee||Marvell International Ltd.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (15), Non-Patent Citations (1), Referenced by (3), Classifications (11), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a continuation application of U.S. application Ser. No. 11/463,278, filed Aug. 8, 2006, which is a divisional application of U.S. application Ser. No. 10/737,648 filed Dec. 15, 2003, now U.S. Pat. No. 7,116,504, which claims the benefit of Provisional Application No. 60/457,613, filed Mar. 25, 2003 and Provisional Application No. 60/472,073, filed May 20, 2003. This application incorporates these applications by reference.
1. Field of the Invention
This invention relates in general to the field of information storage, and more particularly to baseline wander correction and reduction of dc noise components in Hard Disk Drive (HDD) read channels.
2. Description of the Related Art
Hard disk drives (HDD) typically comprise at least one disk having a magnetic medium for storing information, a spindle, a controller for controlling disk rotational speed, a transducing head (for reading and writing data), a servo actuator assembly including a positioning controller for positioning the head over the appropriate disk track, and data channels for transmitting data to and from the disk. The transducing head reads data from and writes data to the disk in data blocks having either fixed or variable length. A data block comprises a preamble (for acquiring timing signals), timing bits, a position error field, address bits, data bits, and error correction bits. Data blocks are recorded in sectors in concentric tracks. A track may comprise several sectors. The number of sectors may depend on the radial location of the track on the disk.
Data channels on HDD transmit and receive data via a communication medium. HDD and communication systems utilize similar techniques to encode data for storage and retrieval or for transmission and reception wherein data is encoded into a form in which it may be easily manipulated. Most modern applications of such systems encode data as numeric or digital information, wherein discrete numeric values are used to represent user data.
The storage or communication media do not directly manipulate such digital data. Rather, these media store or transmit analog signals representative of the digital data. For example, encoded digital information may be represented as magnetic flux transitions stored on a magnetic disk of an HDD. Two known ways of recording data magnetically on a disk are longitudinal recording and perpendicular recording. In longitudinal recording, as the name implies flux transitions are recorded side by side in a lengthwise fashion. In perpendicular recording the flux transitions are “stood on end,” as it were. Details of implementation of longitudinal and perpendicular recording are very well known to ordinarily skilled artisans, and so need not be detailed here.
As disk drive data densities have increased, the need to put more flux transitions has become more acute. Perpendicular recording has been known for some time. However, problems associated with performing perpendicular recording have prevented its adoption.
Data recorded on a disk may be retrieved to decode the signals and reproduce the encoded digital data. A read channel reads or receives the encoded data and reproduces the original digital signal. In general, a read channel includes a transducer component that senses the analog signal and digital processing components that detect sequences of changes in the signal that represent encoded digital data. For example, a read channel used in a magnetic storage device includes a transducer head that senses the magnetic flux transitions and produces a continuous analog signal that must then be detected and decoded. In general, the shape of the continuous waveform represents the encoded digital data.
Analog signal processing circuitry, such as amplifiers, filters, and converters, introduces a dc offset that varies as a function of temperature and signal gain resulting in a signal level shift which, if uncorrected can degrade performance of the data recovery system. The dc offset raises or lowers the normal peaks and valleys of the analog signal and the changing levels of the dc offset cause baseline wander that adversely affects the read channel gain, stability and overall quality.
Varying dc offset causes baseline wander, which is low frequency disturbance of a radio frequency signal causing variations in the peak values of a signal. This dc offset can be a problem for both longitudinal and perpendicular recording. In perpendicular recording, the main cause of baseline wander in perpendicular recording is the pre-amplifier and the read channel are ac coupled. The ac coupling acts as a high pass filter. Since the perpendicular recording read back signal contains significant low frequency components, the high pass filter introduces baseline wander.
One known approach to canceling the dc offset is to block dc. Typically, blocking uses ac coupling, wherein a coupling capacitor is placed at the output of the analog circuit element. Blocking can lower dc offset arising on the analog input line. However, it does not remove any dc offset caused by digital signal processing. Thus, blocking is used on the communication line outside the read channel. One disadvantage of dc blocking is that the capacitors necessary for blocking the dc offset occupy considerable chip space, thereby making blocking unsuited for applications when a read channel is implemented on an integrated circuit. Accordingly, blocking is not a very effective technique for handling dc offset in a read channel.
Another approach, shown in
The error signal enters a loop filter 211, and is biased with a constant loop gain, μ1, and then is summed with the current value of the dc correction signal. The current value of the dc correction signal results from delaying the biased error signal. The dc correction signal is converted to an analog signal with a digital to analog converter (DAC) 228 and then is summed with the filtered input signal.
Copending, commonly assigned application Ser. No. 09/053,612 filed Mar. 27, 2000, incorporated herein by reference, shows still another approach wherein the dc correction circuitry has a first and a second feedback loop. Referring now to
A second summer combines the error signal, e0, and the output of the first feedback loop thereby restoring the first error correction signal. The output of that second summer is input to a second dc feedback loop, including a second loop filter 313. In the second loop filter 313, a second biasing circuit combines the restored first error correction signal and a second loop gain constant, μ2, to form a second product that is an input to a third summer circuit. The third summer circuit is additionally connected to the output of the second dc feedback loop to receive a current second dc offset cancellation signal. The third summer circuit adds the second product with the current second dc offset cancellation signal to produce a next second dc offset cancellation signal. A second accumulator is connected to the third summer circuit to produce the second dc offset cancellation signal. The second dc offset cancellation signal then passes to an digital-to-analog (DAC) converter 328 and is then recombined with the input signal.
The present invention is directed to improved signal processing circuitry for reducing baseline wander and dc offset. To address the stated need and fulfill other desired objectives, in accordance with the invention, a communications read channel provides circuitry for removing dc components from a signal acquired by the read channel. One or more loop filters may be provided, providing corrections to a digital signal, and optionally to the analog input signal. The invention is applicable to HDDs using either longitudinal and perpendicular recording.
In a read channel for a magnetic recording system, a bit detector might be sensitive to a dc offset of the input signal when matched to a target that is not dc free. The analog circuitry and the high-pass filtering of a signal from perpendicular recording media are common sources of dc offsets. The dc offset caused by the analog circuitry is normally stationary and changes very little during a sector of data. The high-pass filtering due to lower high-pass cut-off corners used in perpendicular recording results in a faster varying dc offset. Therefore, the inventive circuit shown in
In general, the inventive circuitry provides one or more feedback loops to correct the dc offset in the digital domain or in both the digital and analog domains. Each of the feedback loops comprises a loop filter having one or two loop inputs. The first type, shown in
The uncorrected digital signal, y, is delayed through timing loops 412 and 414 (each timing loop 412 and 414 providing a delay of L1 and L2 cycles respectively) and is subtracted from the output of the first output of the first detector 408, y′″, to form an error signal e0. The delay L1 introduced by the first timing loop 412 can be 0, 1 or greater so long as the sum of the delays of L1 and L2 equals the total delay caused by he detector 408. The error signal e0 is provided to the first loop input.
A simple detector 410 may also be provided. The simple detector 410, preferably having a small latency, may be a detector such as a decision feedback detector, peak detector, or threshold detector. Such detectors are well known in the art. The simple detector 410 may also include a reconstruction filter such as an FIR filter. The simple detector 410 may provide one or two outputs, the first output is provided by the reconstruction filter while the second output is the output of the detector before the reconstruction filter.
The second loop input may be one of several signals: the first or second output of the detector, the corrected digital signal y′, the first or second output of the simple detector, or the uncorrected digital signal y. When one or more of the signals are present for selection as the second loop input, a selector 422 may also be provided. The selector 422 may be an active device or may be wiring choice.
The loop filter 418 provides the digital dc offset correction signal zd. The correction signal zd also corrects the first correction loop after being delayed L2 cycles by delay loop 414. Because the digital dc offset correction applied to the loop filter is only delayed L2 cycles rather than the total delay, the response time of the correction loop is shortened.
The third loop filter 720 output combines with the second portion of the second loop filter 718 output to provide an analog dc offset correction signal za. The analog dc offset correction signal, za, passes through the digital-to-analog converter 728 and combines with the analog input signal x to form the corrected analog input signal x′.
The signal to the fifth loop input may be supplied by a second selector 824 that operates independently from the first selector 822. Each selector 822, 824 may be an active device or may be a wiring selection as appropriate.
The foregoing description depicts both an apparatus for removing dc offset, and a method for removing dc offset. The invention may be embodied in a computer product containing program code for performing the inventive method. Examples of such computer products include, but are not limited to many different types of removable and fixed media which are well known to ordinarily skilled artisans.
Therefore, the foregoing is illustrative only of the principles of the invention. Further, those skilled in the art will recognize that numerous modifications and changes are possible, the disclose of these embodiments does not limit the invention to the exact construction and operation, and accordingly, all suitable modifications and equivalents fall within the scope of the invention.
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|U.S. Classification||360/39, 327/307, 375/341, 360/46, 375/350, 375/319|
|Cooperative Classification||G11B20/10046, G11B20/10203|
|European Classification||G11B20/10A6C, G11B20/10A6|